Merkel Cell Carcinoma (MCC) is a neuroendocrine skin cancer that is diagnosed in 1500 Americans each year and has a disease-specific mortality of 46%. Multiple lines of epidemiologic and clinical data strongly link environmental ultraviolet (UV) exposure to the development of MCC. One role of UV that may contribute to the development of MCC is its immune suppressive action in the skin. Other risk factors for MCC include immune system dysfunction (e.g. HIV, organ transplant medications, chronic lymphocytic leukemia), advanced age, and a recently discovered polyomavirus (the Merkel cell polyomavirus, MCPyV). Despite the rising number of deaths from MCC and the increasing body of evidence acknowledging the importance of the immune system in preventing and controlling this cancer, the nature of the cellular immune response to the tumor remains largely uncharacterized. We hypothesize that persistently expressed MCPyV viral proteins are immunogenic to T-cells and we will investigate how this tumor escapes immune system surveillance in an immune competent host. In Aim 1, we will define the specificity, phenotype and effector function profile of the cellular immune response to two representative MCPyV antigens in MCC patients. These studies will contribute to an ongoing effort in our lab to target MCPyV for rational immunotherapy. Extending our preliminary data, we will characterize two immune escape mechanisms that may be important for MCC initiation and progression. Studies in Aim 2 will investigate immune tolerance by antigen specific regulatory T cells (a subset of which may be UV-induced). The mechanisms and reversibility of Major Histocompatibility Complex class I (MHC-I) down-regulation will be explored in Aim 3. The studies in this proposal will capitalize on a rich repository of clinically annotated human tumor and blood samples. We are currently following the clinical status of over 300 MCC patients and in many of these cases we have blood and/or tissue samples that are archival, frozen or fresh. In addition, multi-disciplinary collaborations (including with an immunologist focused on viral skin infections) have been established to train the applicant in the relevant disciplines of immunology, molecular and cancer biology, as well as epidemiological and clinical studies pertinent to the pathogenesis of Merkel cell carcinoma. This interdisciplinary training environment is thus ideally suited for preparing the applicant for a career as a physician-scientist. In summary, these proposed translational studies will characterize MCPyV-reactive T cells and immune evasion mechanisms employed by a UV-associated skin malignancy, which is an important step for developing promising immune-based therapy for Merkel cell cancer patients.